On Wed, 2010-12-01 at 15:41 -0500, John Foreman wrote:
> Hello,
>
>
>
> Are there any plans to put any functions in GMPL that take a set as
> input and return an individual element? I’m thinking of functions such
> as element(set,index), first(), and last() in AIMMS and OPL.
>
>
>
> Thanks,
>
> John Foreman
>
> Senior Operations Research Consultant
>
>
>
> Revenue Analytics, Inc.
>
> 3100 Cumberland Blvd.
>
> Suite 1000
>
> Atlanta, Georgia 30339
>
> www.revenueanalytics.com
>
>
Set is an unordered collection, so it is impossible to define any
natural ordering of its elements. For example, the first element of a
set could be defined as an element which appears first in some list,
however, it could also be defined as a lexicographically minimal
element, or even as an element on which some ordering function takes a
minimal value.
In GNU MathProg this issue can be resolved by defining a mapping like
follows:
set S;
param elem{i in 1..card(S)}, symbolic, in S;
data;
set S := red green blue;
param elem [1] blue, [2] red, [3] green;
where elem[i] is i-th element of S.
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